The invention relates to a method for synchronizing optical signals conducted via different optical waveguides to an optical network node, wherein variations in propagation time of the optical signals are compensated by at least two adjustable optical delay circuits, an optical synchronizing device for implementing an adjustable delay for the optical signals of an optical waveguide, an optical synchronizing device for implementing adjustable delays for the optical signals of a plurality of optical waveguides and an optical network node comprising at least one synchronizing device.
Present-day communications transmission technology is characterized by two main directions of development. The first development direction leads from electrical transmission devices to optical transmission devices and encompasses the construction of increasingly complex network nodes, for example multiplex devices or switching devices, such as to obviate the need for the conversion of the optical signals into electrical signals. The second development direction leads from continuous data signals to packet-oriented data signals. Whereas, at least in commercial systems, packet or cell switching currently is still performed exclusively for electrical signals, high-speed optical networks with optical packet switching are already being successfully implemented in research projects, such as for example the research project xe2x80x9cKeys to Optical Packet Switchingxe2x80x9d (KEOPS) promoted by the European Community and referred to in the following as KEOPS project. The main network elements of such an (optical) network are represented by (optical) packet switching nodes, the function of which is to identify the packets contained in the received (optical) signals, analyze their control information, and forward the packets to further (optical) network nodes in accordance with their control information. To ensure a simple and effective switching of the packets, the signals which are transmitted via different optical waveguides and brought to the packet switching node must be synchronized. For this purpose the packet switching node is preceded by a synchronizing device which contains an adjustable delay circuit for each optical waveguide. Here a delay circuit consists for example of a cascade of optical switches by means of which different individual delay elements or delay lines, i.e. optical waveguides of a specific length, are connected to form a resultant delay line leading to a specific delay of the optical signal conducted across the delay circuit.
The synchronization fundamentally takes place in two different phases. In a first phase prior to the operation of the network, the so-called start-up phase, a basic setting takes place, in the following also referred to as static synchronization, which compensates for the different lengths of the optical waveguides. As the switch-over behavior of the optical switches is non-critical in this phase, slow optical switches can be used to set a corresponding delay circuit. In a second phase, the so-called operating phase, propagation time changes which occur during operation are compensated by a so-called dynamic synchronization. These propagation time changes or propagation time variations are caused in particular by thermal effects. As soon as a specific change in the propagation time of an optical waveguide is measured through the identification of the synchronization patterns in the signals, the corresponding delay is adapted by a new interconnection of delay elements. Here the transmission of the optical signals should not be disturbed such as to lead to the destruction of one or more optical packets. Therefore a switch-over from one delay value to another should only take place in short, defined time pauses, the so-called guard time, and must be completed within this time; the switches used must therefore possess a switching time which is shorter than the said guard time.
As on the one hand large delay variations must be compensated, but on the other hand a high resolution is also required, a specific number of delay lines must be able to be connected by means of a cascade of optical switches. Here an optical signal passes through all of these switches. Depending upon the switch technology, each optical switch gives rise either to a specific loss of optical power or, in the case of switches with integrated optical amplifiers, an increase in noise power. In this way, for example in the case of so-called semiconductor optical amplifier (SOA) gates, a critical reduction in the optical signal/noise ratio (OSNR) can occur through the series connection of a larger number of the switches. However, due to the required fast switching time it is not possible to use virtually loss-free switches, for example micro-electronic mirrors (MEM).
Commencing from a method of the above described type, the object of the invention is to indicate a means whereby the required number of high-speed optical switches of a synchronizing is device can be reduced to a minimum.
This object is achieved in accordance with the invention by a method for synchronizing optical signals conducted via different optical waveguides to an optical network node, wherein variations in propagation time of the optical signals are compensated by at least two adjustable optical delay circuits, an optical synchronizing device for implementing an adjustable delay for the optical signals of an optical waveguide, an optical synchronizing device or implementing adjustable delays for the optical signals of a plurality of optical waveguides, and an optical network node comprising at least one synchronizing device.
Apart from possibly occurring jitter effects, in particular limited propagation time variations occurring in the packet timing, the above described propagation time variations have a relatively slow temporal course. The jitter effects are compensated by special fast synchronization devices which will not be considered in detail here. The remaining slow propagation time variations thus do not require frequent switching over of the corresponding synchronizing device. The basic concept of the invention is to set the delay of an initially passive delay circuit (off-line), for which purpose a relatively large amount of time is available. Then this delay circuit is switched into the active state. A delay circuit according to the invention can thus in principle be constructed from two delay circuits each comprising only slow optical switches which are switched over by a high-speed optical switch. Only this switch-over process needs to take place with a fast switching time within the above mentioned guard time.
An advantageous embodiment of the invention consists in that a synchronizing device for a specific number of optical waveguides (e.g. four) comprises a total number of slowly adjustable delay circuits increased by one (e.g. five). Here the additional delay circuit compared to the number of optical waveguides is switched passive in its basic state and is switched active only for the setting of a new delay for an optical waveguide.